Softness, in this chapter, is the sign of being alive; the stiff and hard are what death leaves behind, in a body or in a tree branch equally. Laozi's (say "laow-dzuh") point isn't really about posture, it's about what rigidity costs a living thing: the plant that won't bend breaks in the same wind the supple one survives. You can watch yourself go rigid the same way under a small pressure: the fixed opinion, the plan that has to happen exactly as drawn. Today, notice where you're holding something stiffly out of habit, and let it stay a little more supple instead.
Man at his birth is supple and weak; at his death, firm and strong. (So it is with) all things. Trees and plants, in their early growth, are soft and brittle; at their death, dry and withered. Thus it is that firmness and strength are the concomitants of death; softness and weakness, the concomitants of life.
Tao Te Ching, ch. 76 (trans. James Legge)